1. Field of the Invention
This invention relates to solid state lighting and in particular to solid state light sources or lamps that emit warm light with a high color rendering index (CRI).
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with approximately ninety percent of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs by a factor of about 10, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color reproduction. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in their LED lights being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.
There are other lighting characteristics that are important to the general acceptance of LEDs for some applications. The light generated by different light sources, including incandescent, fluorescent and LEDs, can be measured in terms of color rendering index (CRI or CRI Ra) and color temperature. CRI is a quantitative measurement of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Stated differently, it is a relative measure of the shift in surface color of an object when lit by a particular lamp. Light sources with a high CRI approaching 100 can be desirable in color-critical applications such as photography and cinematography. Daylight has a high CRI of approximately 100 and incandescent bulbs have a relatively close CRI of greater than 95. By comparison, fluorescent lighting has a lower CRI in the range of 70-80, and mercury vapor or sodium lamps have a much lower CRI of 40 or less. High quality light suitable for general indoor illumination should have a CRI of greater than 90.
Color temperature is a characteristic of light source that is determined by comparing the light's chromaticity with that of an ideal black-body radiator. The temperature (usually measured in kelvins (K)) at which the heated black-body radiator matches the color produced by the light source is that source's color temperature. For incandescent light sources the light is of thermal origin and is very close to that of an ideal black-body radiator. Higher color temperatures of 5000 K or more are “cool” and have green to blue colors while lower color temperatures of 2700 to 3500 K are considered “warm” and have yellow to red colors. General illumination can have a color temperature between 2,000 and 10,000 K, with the majority of general lighting devices being between 2,700 and 6,500 K.
In contrast to incandescent radiation, light sources, such as fluorescent lamps, emit light primarily by processes other than raising the temperature of a body. This means the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp. For high quality light sources it is also important that color of the illumination be as close as possible to that of a black body spectrum (i.e. black body locus on CIE chromaticity diagram).
Solid state light sources have been developed that utilize a one or more LEDs coated by a conversion material to produce a white wavelength of light. Some of these include blue emitting LEDs covered by a conversion material such as YAG: CE or Bose that absorbs some of the blue light and emits a yellow/green light. These LEDs emit a white light combination of blue LED light and yellow/green conversional material light. White light can similarly be produced with blue or UV LEDs covered by RGB phosphors, with the LEDs emitting a white light combination of red, green and blue. These methods have generally good efficacy, but only medium CRI. These have not been able to demonstrate both the desirable high CRI and high efficacy, especially with color temperatures between 2700K and 4000K.
Techniques for generating white light from a plurality of discrete light sources to provide improved CRI at the desired color temperature have been developed that utilize different hues from different discrete light sources. Such techniques are described in U.S. Pat. No. 7,213,940, entitled “Lighting Device and Lighting Method”. In one such arrangement a 452 nm peak blue InGaN LEDs were coated with a yellow conversion material, such as a YAG:Ce phosphor, to provide a color that was distinctly yellow and had a color point that fell well above the black body locus. The yellow emission was combined with the light from reddish AlInGaP LEDs that “pulled” the yellow color of the yellow LEDs to the black body curve to produce warm white light. FIG. 1 shows a CIE diagram 10 with the tie lines 12 between red light 14 from red emitting LEDs and various yellow and yellowish points on the blue/YAG tie line 16. With this approach, high efficacy warm white light can be produced with improved CRI. Some embodiments exhibited improved efficacy, with CRI Ra of greater than 90 at color temperatures below 3500 K. The LR6, LR4 and LR24 LED based lighting fixtures, commercially available from Cree, Inc. (wwww.cree.com), can emit light with these improved characteristics.
In different lighting applications it can be desirable to use a light temperature of light that can appear more yellow or orange. Some of these can include illuminated areas outside of commercial establishments such as restaurants or retail locations. These yellow or orange colors have typically been provided by sodium vapor lights which can be undesirable because of their low CRI. This yellow/orange color can also be provided by incandescent bulbs utilizing the appropriate filter, but as mentioned above, these bulbs have a short lifetime and are very inefficient. The efficiency is made even worse by use of a filter.